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The inhibition of mild steel corrosion in sulphuric acid solution by Gmelina arborea root extract was studied using weight loss and hydrogen evolution methods. The results obtained reveal that the extract appreciably inhibited the corrosion of mild steel in the acid medium. The inhibition efficiency increased with increase in Gmelina arborea root extract concentration but decreased with increase in temperature. The highest inhibition efficiency of 78.59% occurred at extract concentration of 2 g/L at 30°C by weight loss measurements. Physical adsorption has been proposed for the adsorption of Gmelina arborea root extract onto the mild steel surface. The calculated thermodynamic parameters revealed that the adsorption process was endothermic and spontaneous. The phytochemical screening of Gmelina arborea root extract revealed the presence of alkaloids, saponins, anthraquinones and terpenes. The adsorption of the root extract on mild steel surface obeyed the Langmuir adsorption isotherm.